A field-emission type electron gun is a cold-cathode electron gun which emits electrons by a field effect, and it is an important element of a micro vacuum device such as a vacuum switching device, vacuum amplifier device, micro display device or the like.
C. A. Spindt et al., Journal of Applied Physics, Vol. 47, No. 12, pp. 5248-5265, 1976 reports a field-emission type electron gun in which an emitter is made of molybdenum. However, since this type of electron gun requires that molybdenum cone is formed on a conductive substrate, a high-precision processing thereof is too difficult.
Recently, various methods for forming the emitter by employing silicon with good processability are suggested. For example, Japanese patent application laid-open No. 4-94033 discloses a method (hereinafter referred to as "first prior art") as shown in FIGS. 1A to 1D and 2A and 2B.
First, as shown in FIG. 1A, silicon dioxide film 2 is deposited on a silicon substrate 1 of, for example, N-type. Thereafter, as shown in FIG. 1B, the silicon dioxide film 2 is patterned by photolithography to leave a part of the silicon dioxide film 2 where an emitter will be formed.
Next, as shown in FIG. 1C, the silicon substrate 1 is isotropically etched to form a convex portion, followed by applying thermal oxidation to the surface of the silicon substrate 1 to form silicon dioxide film 3 thereon as shown in FIG. 1D. Due to this step, the convex portion of the silicon substrate 1 is sharpened to form a conical emitter 1a.
Thereafter, as shown in FIG. 2A, insulating film 6 made of, for example, silicon dioxide is deposited by the deposition method, followed by depositing film 4a for gate electrode by, for example, deposition method to form a gate electrode 4 thereon. Thereafter, as shown in FIG. 2B, the silicon dioxide films 2, 3 and insulating film 6 on the emitter are etched by hydrofluoric acid to lift-off the film 4a for gate electrode over the emitter region to expose the emitter 1a. In this method, to form the emitter made of silicon and the gate, the deposition method and lift-off method are employed.
Hereon, Japanese patent application laid-open No. 6-52788 discloses that a gate electrode is formed at a concave portion by the etch-back method in place of the lift-off method in the first prior art.
On the other hand, Japanese patent application laid-open No. 3-222232 discloses another method( hereinafter referred to as "second prior art") as shown in FIGS. 3A to 3E.
First, as shown in FIG. 3A, photoresist film 7 in which an opening is formed by photolithography at the region where an emitter will be formed is formed on a silicon substrate 1 with (100) face orientation. Using the photoresist film 7 as a mask, the surface of the silicon substrate 1 is then etched by an etchant such as tartaric acid, sulfuric acid to form a conical or V-shaped groove thereon.
Next, as shown in FIG. 3B, the photoresist film 7 is removed, followed by forming tungsten film to provide an emitter electrode on the silicon substrate 1. Thereafter, as shown in FIG. 3C, the silicon substrate 1 is polished from the back surface of the silicon substrate 1 toward below the emitter electrode. Subsequently, as shown in FIG. 3D, the silicon substrate 1 is further thinned by polishing or wet-etching to expose the tip of the emitter electrode 8.
Thereafter, silicon dioxide film 9 is deposited on the back surface of the silicon substrate 1, and photoresist is coated thereon and etched back to expose a part of the silicon dioxide film 9 on the tip of the emitter electrode 8, followed by selectively etching the exposed part of the silicon dioxide film. Finally, after forming metal film of aluminum etc. on the silicon dioxide film 9, a grid electrode 10 and anode electrode 11 are formed by photolithography and dry-etching to obtain an electron gun as shown in FIG. 3E. In the above method, the exposing of the emitter electrode is by both the polishing and etch-back method.
In general, to well control electrons emitted form an emitter, such a field-emission type electron gun has to be formed such that the distance between an emitter and a gate is sufficiently shortened and the height difference between the emitter and the gate is within a given range. Furthermore, in mass production, the plane uniformity of a wafer as well as the reduction of fluctuation in quality between wafers is required. Thus, it is desired that the positioning of the tip of an emitter and a gate is self-aligned.
In the first prior art, though the gate electrode is formally formed self-aligned, the distance between the emitter and the gate can not be sufficiently shortened and the height of the gate can not be precisely controlled. In the first prior art, the distance between the emitter and the gate is determined by a mask size of the silicon dioxide film 2 for forming the emitter. However, this size can not be optionally decreased because it is an important factor to determine the height and shape of the emitter cone. Moreover, it is difficult to constantly keep the height difference between the emitter and the gate electrode since the respective thicknesses of the silicon dioxide film 3 and insulating film 6 by which the height of the gate electrode is determined may fluctuate in the film formation process thereof.
In the second prior art, since the grid electrode corresponding to a gate electrode is not formed self-aligned, it is difficult to constantly shorten the distance between the emitter and the grid without fluctuation. Furthermore, in the second prior art, it is difficult to constantly keep the height difference between the tip of the emitter and the grid electrode 10, since the thickness of the silicon substrate may fluctuate when it is polished because the stopper for the polishing does not exist, and further, the thickness of the silicon dioxide film may fluctuate. Moreover, the tip of the emitter may be damaged by an error in the process that the silicon substrate 1 is polished.